Firstly, taking an example of boiler tube, in the past days when the operation temperature of the boiler was lower than that in these days, and erosion/corrosion environment inside the furnace was not so severe, the steel tube for boiler (low alloy steel tube) was commonly used without coating taking into consideration of high temperature usability and mechanical characteristics. Although stainless tube and titanium tube were also used for the application of requiring corrosion resistance, such uses were not common because of the high costs.
In recent years, boilers recovering and utilizing refuse incineration heat have increased, problem of erosion (wear) is caused by combustion ash dust. In order to solve this problem, boilers with a specification applying thermal spray coating of self-fluxing alloy (first alloy material) with high erosion resistance are starting to be commonly used.
However, the above described self-fluxing alloy coating is the coating which is left as thermal-sprayed by, for example, HVOF (High Velocity Oxygen Fuel) thermal spraying apparatus (that is, the coating is porous having pinholes reaching base metal herein after referred to as “unmelted coating”); a self-fluxing alloy coating subjected to the melting process after thermal spraying is rarely used although melted coating is commonly used in other application (for example, rollers for metal sheet processing line) (which is modified from the porous coating into the dense coating, and provided with sufficient environment blocking function without pinholes, and is the example of the “melted coating”).
The reason why the application of the melted coating of the self-fluxing alloy for the boiler tube is uncommon is that an extraordinary difficult work is required because the thermal shock cracking easily occurs to the boiler tube on its melted coating under rapid local temperature rise at the time of the welding operation when the boiler tube is welded to connect, the whole tubes have to be preheated inside the furnace, then connected by welding at high temperature.
However, as for the boilers in recent years, the problems concerning not only erosion but also corrosion are becoming important together with the demand for high-temperature burning to make the exhaust air harmless, thus melted coating application to the boiler is increasingly desired in the form of provision of prefabricated coating portion.
As one example thereof, a constitution that adopted the melted coating of the self-fluxing alloy to the boiler tube is disclosed in the patent document 1 (Japanese Patent Application Laid-Open (JP-A) No. H10-170194). There the constitution provides a non-coated portion of about 50 mm at an end portion of the boiler tube, excluded from the thermally sprayed coating where the non-coated portion is used as a part for the joint (the patent document 1, page 3, the fourth column, lines 15 to 16). Further, a process of fitting a protector member on the above non-coated portion is added instead of the coating, after the welding (Id. lines 24 to 26). The above process requires a special order of the protector member (for example, the protector member made of alumina) having high erosion resistance, or fitting operation inside narrow boiler. Therefore, the process results in higher cost in material and work, and also requires front loaded procurement of the materials.
As for another approach, one may imagine a method that adopts welding to connect prefabricated boiler tubes having unmelted coating of a self-fluxing alloy, at the construction site, and then apply the melting process by an induction heating or the like at that location. However, it is practically impossible due to the narrow space or difficulty in heating of the interfaced portion with the other members.
By the way, since, reheating-crack occurs unless the entire simultaneous melting or one direction melting is performed in the melting process of the self-fluxing alloy, the induction heating is indispensable for such an execution at the construction site.
Next, in the case of the boiler furnace panel, since it is composite constitution in which tube materials and plate materials are arranged alternately as described above, or since it has large dimension (for example, 0.5 m×6 m), use of a practical supplemental member corresponding to the above protector is more difficult. Further there is the problem of the complicated shape distortion associated with the melting process after the thermal spraying (see the patent document 2 or the patent document 3). Thus utilization of the prefabricated melt-coating product itself has been difficult to consider under such circumstances.
Patent Document 1: JP-A No. 10-170194
Patent Document 2: JP-A No. 2001-4101
Patent Document 3: JP-A No. 2000-329304